An engine may be supplied fuel by both port and direct fuel injectors. The port fuel injectors may provide advantages during cold engine starting, and the direct fuel injectors may provide advantages when the engine is operated at higher speeds and loads. For example, during cold engine starts, directly injected fuel may impinge on engine pistons where soot may form, thereby increasing engine particulate matter output. However, if fuel is port injected, the injected fuel may evaporate as it is being drawn into engine cylinders so that less particulate matter is formed. At warmer temperatures, directly injected fuel may cool cylinder charge mixtures so that an engine may be less prone to knock at higher engine speeds and loads during warm engine operating conditions. Consequently, directly injected engines may exhibit improved fuel economy and improved performance. Additionally, it may be desirable to operate both direct injectors and port injectors during some operating conditions to improve combustion stability and engine emissions.
Thus, it may be beneficial to incorporate port and direct fuel injectors into an engine. However, supplying fuel via two different injection systems may make it difficult to ascertain which fuel injection system is providing more or less fuel than is desired during some operating conditions. Determining which injection system is providing more or less fuel than is desired may be particularly difficult when both injection systems are providing fuel to the engine. Therefore, it may be desirable to be able to determine which fuel injection source may be introducing fueling errors to the engine.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for fueling a cylinder, comprising: injecting fuel to the cylinder via a first fuel injector and a second fuel injector; and indicating degradation of the first fuel injector or the second fuel injector in response to a rate of change of air-fuel ratio error and a fraction of fuel injected via the first fuel injector or the second fuel injector.
By allocating portions of an air-fuel error based on fractions of fuel injected to a cylinder, it may be possible to provide the technical result of differentiating fueling errors from one fueling system in a system where two fueling systems provide fuel to engine cylinders. For example, engine air-fuel ratio errors may be determined via a difference in a commanded air-fuel ratio and air-fuel ratio as determined from an oxygen sensor. And, a portion of the air-fuel ratio error may be allocated to a direct fuel injection system by dividing a change in air-fuel ratio error by a change in a fuel fraction provided by a direct injection fuel system. Likewise, a portion of the air-fuel ratio error may be allocated to a port fuel injection system by dividing the change in air-fuel ratio error by a change in a fuel fraction provided by a port fuel injection system. In this way, it may be possible to determine which of two fueling systems may be contributing greater fueling errors to engine air-fuel ratio control.
The present description may provide several advantages. In particular, the approach may reduce engine air-fuel error. Further, the approach may make it possible to direct service personnel to one of two separate fuel systems during conditions of fuel system degradation. Further still, the approach may provide for increased operation of a first non-degraded fuel system in the presence of a second degraded fuel system.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.